In order to improve the wear resistance or thermal resistance characteristics of a surface of a substrate, it has been proposed to apply to the surface a material such as silicon carbide. The prior art has suggested that silicon carbide be applied in a plasma spray operation to the surface of a substrate so as to provide a protective coating therefor. However, when a person plasma sprays silicon carbide onto the surface of a substrate, the deposition products include silicon carbide, silicon dioxide and a very minute amount of silicon not exceeding parts per million. The coating, so applied, is not tightly adherent to the surface of the substrate and may be removed easily by wiping or scraping.
We, in our own experimentation, have documented the fact that silicon carbide cannot be successfully plasma sprayed onto the surface of a substrate. We have found, as others have found, that arc spraying of silicon carbide does not produce a tightly adherent coating on a substrate. Rather, such arc spraying produces a coating on the substrate which can be removed easily by either a wiping action or a scraping action. Thus, the arc plasma spraying of silicon carbide onto a substrate does not produce an article of manufacture which has utility in providing either a wear resistant or a heat resistant surface on the substrate.
U.S. Pat. No. 3,419,415 teaches the plasma spraying of a mixture containing a refractory carbide and at least five percent by weight, based on the molecularly bound carbon in the carbide, of excess carbon in a form capable of reacting with the metal of the refractory carbide at flame sprayed temperatures to form a carbide. This patent indicates that the excess carbon is preferably in an amount of 10 to 100% by weight of the molecularly bound carbon. The patent mentions that silicon carbide may be one of the starting carbide materials.
We attempted to reproduce the process disclosed in U.S. Pat. No. 3,419,415 according to the patent teachings while using silicon carbide as one of the ingredients of the mixture to be arc sprayed. The other portion of the mixture was carbon. In two separate experiments carried out on the silicon carbide-carbon mixture, the results achieved by us did not live up to what was stated in the patent. In both experiments, the coating deposited on the substrate contained silicon carbide and silicon oxide, as well as minute traces of silicon in the range of parts per million. The adherence of this coating to the substrate was in the range from no adherence to a very low level of adherence. The deposited coatings could be wiped or scraped from the substrate surface with little or no trouble and, therefore, did not provide on the surface either a wear resistant or a thermal resistant coating. The results achieved by spraying a combined silicon carbide-carbon mixture, in our opinion, was the same as is achieved when silicon carbide is sprayed by itself.
In order to duplicate the coating procedure set forth in U.S. Pat. No. 3,419,415, as above described, we conducted a pair of experiments, as described hereinbelow.
In the first experiment, 40 cc of beta silicon carbide powder was mixed with 12 cc of graphite, 4 cc of furfural and 50 cc alcohol, forming a coating compound where each individual silicon carbide particle was coated with a thin carbon film as per the instructions of the patent. This mixture was dried to form a powder and then plasma sprayed on several different substrates. The deposited coatings were tested for adherence and found to be easily removed from the substrates by a wiping or a scraping action. As stated above, an X-ray defraction pattern taken of some of the coatings showed that the coatings were formed of silicon carbide, silicon dioxide, and minor proportion of silicon in the parts per million range.
In a second experiment, commercial grade graphite was mixed with silicon metal powder and then plasma sprayed onto different substrates. This mixture contained 33% by volume graphite in 66.6% by volume silicon metal powder. The deposited coating was tested for adherence and was found to be removed easily from the substrate by wiping or scraping. Once again, the X-ray defraction characteristics of the coating indicated that the coating was made up of silicon carbide, silicon dioxide and a very minor proportion in parts per million of silicon. Thus, the U.S. Pat. No. 3,419,415 patent teaches a silicon carbide-carbon coating which is not tenaciously adherent to a substrate when flame sprayed thereon.
U.S. Pat. No. 3,274,077 discloses a high temperature resistant, self-healing coating and method of application. Generally, the method relates to the arc plasma spray coating of a coating which consists of a refractory oxide having silicon mixed therewith. In this particular case, the material which is arc plasma sprayed produces a coating on a substrate in which the silicon coats over and between the refractory oxide particles. The resulting coating on the article is one in which refractory oxides are associated with silicon. There is no interaction between the refractory oxide and the silicon associated therewith.
It is an object of the present invention to provide as a new article of manufacture, a substrate having a tightly adherent coating thereon which is wear resistant and heat resistant, the coating consisting essentially of beta silicon carbide and silicon.
It is a further object of this invention, to provide a new coating composition for arc plasma spraying which consists essentially of finely divided silicon carbide mixed with finely divided silicon.
It is a still further object of this invention, to provide a method of developing a coating on a substrate in which a mixture of finely divided silicon carbide and silicon are fed to a plasma spray and that plasma spray is directed at an exposed surface of a substrate so that the exposed substrate is coated with a coating consisting essentially of beta silicon carbide and silicon.